Implementing Network Admission Control

This chapter is from the book

Self-Defending Networks: The Next Generation of Network Security

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This chapter is from the book

This chapter is from the book 

Self-Defending Networks: The Next Generation of Network Security

Learn More Buy

Operational Overview

This section describes how NAC determines admission compliance and how it then uses the network to enforce the policy to endpoints.

Network Admission for NAC-enabled Endpoints

This section describes the process in which a noncompliant endpoint device is discovered and is denied full access until it is compliant with the admission policy. This scenario is shown in Figure 6-2.

Figure 6-2 Admission Process for Noncompliant Endpoint

The following list is a summary of the admission process for a noncompliant endpoint shown in Figure 6-2:

1. An endpoint attempts to access the network.
2. The NAD notifies the policy server (Cisco Secure ACS) that an endpoint is requesting network access.
3. Cisco Secure ACS checks the NAC policy to determine whether the endpoint is compliant.
4. Cisco Secure ACS forwards specific information to other partner policy servers.
   1. Identity information is sent to a directory server for authentication validation.
   2. Host credentials are sent to an antivirus policy server for posture determination.
5. Cisco Secure Access uses information from the all-policy servers and decides the endpoints authorization. In this example, the endpoint is not compliant and is assigned a quarantine posture.
6. Quarantine enforcement actions are sent from Cisco Secure ACS to the NAD servicing the endpoint.
7. NAD enforces admission actions and communicates posture to Posture Agent.
8. Posture Agent notifies the user that the endpoint is quarantined.

The following sections explain each step in more detail.

Endpoint Attempts to Access the Network

In step 1, the admissions process begins when an endpoint attempts to access the network. What triggers the process is dependent upon the NAD's capabilities and configuration. The NAD initiates posture validation with Cisco Trust Agent using one of the following protocols:

• EAPoUDP
• EAPo802.1x

The protocol used is dependent upon the NAD to which the endpoint connects. Both of these protocols serve as a communication method between the endpoints using Cisco Trust Agent and the NAD. Cisco Trust Agent gathers credentials from NAC-enabled security applications such as antivirus.

NAD Notifies Policy Server

In step 2, the NAD notifies the policy server (Cisco Secure ACS) that an endpoint is requesting network access. A protected tunnel is set up between the policy server and the endpoints posture agent. Once communication is established, the credentials from each of the posture plug-ins are sent to Cisco Secure ACS.

Cisco Secure ACS Compares Endpoint to NAC Policy

In step 3, Cisco Secure ACS looks at the admission control policy and compares the endpoint credentials to the policy to determine whether it is compliant. It determines which of the following posture states to assign to the endpoint:

• Healthy—Endpoint is compliant; no network access restrictions.
• Checkup—Endpoint is within policy, but an update is available. This state is typically used to proactively remediate a host to the Healthy state or to notify a user that a more recent update is available and recommend remediation.
• Transition—This state became available in NAC phase 2. The endpoint posturing is in process; provide an interim access, pending full posture validation. This state is applicable during an endpoint boot in which all services may not be running or audit results are not yet available.
• Quarantine—Endpoint is out of compliance; restrict network access to a quarantine network for remediation. The endpoint is not an active threat but is vulnerable to a known attack or infection.
• Infected—Endpoint is an active threat to other endpoint devices; network access should be severely restricted or totally denied all network access.
• Unknown—Endpoint posture cannot be determined. Quarantine the host and audit or remediate until a definitive posture can be determined.

Cisco Secure ACS Forwards Information to Partner Policy Servers

In step 4, Cisco Secure ACS can optionally send user login (4a) and credentials (4b) to other policy decision servers. When this is done, Cisco Secure ACS expects to receive authentication status and a posture state from each of the policy decision servers.

In step 4a when NAC L2-802.1x is used, Cisco Secure ACS can send identity information to an authentication server. It confirms that the username and password are valid and returns a passed authentication message to Cisco Secure ACS. If identity authentication fails, no posture is checked and the endpoint fails authentication, resulting in no network access.

In step 4b in this example, an antivirus policy server determines that the device is out of compliance and returns a quarantine posture token to Cisco Secure ACS.

Keep in mind that NAC partner policy servers vary and offer a variety of compliance checks besides antivirus. For example, some vendors offer checking for spyware and patch management.

Cisco Secure ACS Makes a Decision

In step 5, Cisco Secure ACS compares all the posture states and determines which posture is the worst; infected is the worst and healthy is the best. It always assigns the worst state and takes the action for that posture. In this example, the user has passed authentication but the endpoint has been assigned a quarantine posture.

Cisco Secure ACS Sends Enforcement Actions

Cisco Secure ACS takes the actions assigned to a quarantine state. In this quarantine example, they can include the following:

• Enforce quarantine access; this varies based on the NAD.

  - For NADs using NAC-L3-IP, the enforcement actions include a quarantine Access Control List (ACL) being applied to the endpoint.

  - For NADs using NAC-L2-IP, the enforcement actions include a quarantine ACL being applied to the endpoint.

  - For NADs using NAC-L2-802.1x, the enforcement action includes a quarantine virtual LAN (VLAN) being applied to the endpoint device.

• Optionally, the endpoint device may be assigned a URL redirect to the remediation server.
• Optionally, a notification message can be sent to the user, indicating that their device is not compliant and is being redirected for remediation.

NAD Enforces Actions

In step 7, the NAD receives the quarantine policy enforcement from Cisco Secure ACS and responds accordingly. In this example, such a response would be to quarantine the endpoint, enforce an endpoint URL redirect to the remediation server, and send a quarantine message to the posture agent.

Posture Agent Actions

In step 8, the posture agent displays the quarantine message, and the user is redirected to the remediation server.

Actions available vary by NAC partner products. Cisco Secure ACS is capable of sending different application actions from HCAP-compliant policy servers to their specific application plug-ins. This can trigger actions such as the following:

• Force an auto-remediation to a designated remediation server
• Force an auto-patch by instructing the host to download and apply a patch automatically
• Restart a stopped application service

In this example, the endpoint is now quarantined, and the user has been notified by a message. The user can elect to do nothing and remain quarantined, or comply and allow their computer to be updated.

The admission control process can take very little time, as little as milliseconds. The time varies and is based on many factors, including:

• Where the endpoint is located in relation to the policy server and optional partner policy servers
• Where the remediation server is located
• NADs performance capability
• Network bandwidth
• How busy the policy servers are

As shown in Figure 6-3, an endpoint is changing from quarantine to healthy posture state.

Figure 6-3 Admission Process for Endpoint Changing from Quarantine to Healthy State

The following list explains the process shown in Figure 6-3:

9. Endpoint remediated.
10. Endpoint polled for change of compliance.
11. Host credentials gathered from endpoint.
12. Host credentials passed to Cisco Secure ACS.
13. Cisco Secure ACS rechecks the NAC policy to determine whether the endpoint is compliant.
14. Cisco Secure ACS forwards specific information to other partner policy servers.
    1. Identity information is sent to a directory server for authentication validation.
    2. Host credentials are sent to an antivirus policy server for posture determination.
15. Cisco Secure Access uses information from all policy servers and decides the endpoints authorization. In this example, the endpoint is compliant and is assigned a healthy posture.
16. Healthy enforcement actions are sent from Cisco Secure ACS to the NAD servicing the endpoint.
17. NAD enforces admission actions and communicates healthy posture to Posture Agent.
18. Posture Agent can notify the user that the endpoint is healthy. Many businesses prefer that a healthy posture be transparent to the user with no message notification displayed.

Endpoint Polled for Change of Compliance

Once an endpoint has been assigned a posture, it stays in effect and is not checked again until a NAC timer has expired or a posture agent trigger occurs.

The following are configurable timers for NAC:

• Status Query—Ensures that an endpoint remains compliant with the admission policy. The timer begins at policy enforcement for the endpoint; compliance is rechecked after the timer expires. Different Status Query timers can exist for different posture states. A shorter amount of time is beneficial for noncompliant states such as quarantine; the device can be rechecked sooner than a healthy device, in order to regain full network access.
• Revalidation—A time in which the posture remains valid. It can be set lower when an outbreak occurs, to force all endpoints to go through the admission policy process again. This enables endpoints to timeout at different intervals depending on where their timers are, versus forcing all endpoints to go through the validation process at the same time.

  In phase 2 with NAC-L2-802.1x, there is no capability to send a status query from the NAD by way of 802.1x. To overcome this, beginning with version 2 of Cisco Trust Agent, an asynchronous status query capability exists. Cisco Trust Agent can send an Extensible Authentication Protocol Over Lan (EAPOL)-Start to the NAD, or CTA can frequently poll all registered NAC application posture plug-ins looking for a change in credentials. If a change exists, it will trigger an EAPOL-Start signaling for a new posture validation.

  In step 10 of Figure 6-3, the quarantine status query timer has expired.

The NAD is aware that the timer has expired for the endpoint, so it begins rechecking for compliance. The posture agent gathers credentials from the posture plug-ins of NACenabled security applications such as antivirus.

Revalidation Process

From step 11 through step 18, the process is the same as the example described in Figure 6-2. The NAD notifies the policy server (Cisco Secure ACS) that an endpoint requests network access. This time, the Cisco Secure ACS determines that the posture is healthy for all admission checks and that the user login is valid. Authentication is successful, and Cisco Secure ACS assigns the healthy policy.

The NAD receives the healthy policy enforcement from Cisco Secure ACS and responds accordingly by allowing full network access. The timers begin for the healthy state.

The NAD informs the posture agent of the healthy status, but no message is sent to the user this time. The user can now resume normal network activity.

Network Admission for NAC Agentless Hosts

The previous example described the admission process for a NAC-enabled endpoint running a posture agent, such as Cisco Trust Agent. This section describes the process for endpoints that do not have a posture agent.

NAC agentless hosts (NAH) can be accommodated by several methods, as shown in Table 6-2. A NAH exception list and whitelist can be created to identify known endpoints that do not have a posture agent installed and running. The option chosen is dependent upon the NAC Framework component and the NAD enforcement method used.

Table 6-2. NAC Agentless Host Exceptions and Whitelisting

The audit server can be used for NAH in all enforcement methods and is a single centrally managed server. As shown in Figure 6-4, an audit server can be included as a decision policy server for NAH. The audit server can determine the posture credentials of an endpoint without relying on the presence of a posture agent.

Figure 6-4 Admission Control for NAC Agentless Host

The following list explains the process shown in Figure 6-4:

1. An endpoint attempts to access the network. The trigger mechanism is dependent upon the NAD's capabilities and configuration. The NAD attempts to initiate posture validation with the posture agent, but no posture agent (Cisco Trust Agent) exists.
2. The NAD notifies the policy server (Cisco Secure ACS) that an endpoint is requesting network access with no Cisco Trust Agent (CTA) present.
3. Cisco Secure ACS cannot determine whether the NAH is compliant because no posture agent exists. Cisco Secure ACS performs the following:
   1. Assign a transition posture to grant a temporary, limited network access to the agentless host while the audit server is determining the full posture validation. The NAD enforces the transition admission policy.
   2. Notify the external audit server that the NAH is requesting admission.
4. Cisco Secure ACS cannot determine whether the NAH is compliant, so it notifies the audit server using GAME to conduct a scan on the endpoint.
   1. The audit server scans the endpoint. It evaluates the endpoint's software information against the audit server's compliance policy. It determines that the operating system patch level is compliant or healthy, but the posture agent is missing, so it is considered noncompliant.
   2. Quarantine is the application posture token (APT) assigned by the audit server for this NAH and is communicated to Cisco Secure ACS.
5. Cisco Secure ACS uses quarantine as the final posture, which is referred to as the system posture token (SPT), and takes the actions assigned to a quarantine state. The actions can include the following:

   - Enforce quarantine access—This varies based on the NAD.
   

   For NAC-L3-IP, the enforcement actions include a quarantine ACL being applied to the endpoint.

   For NADs using NAC-L2-IP, the enforcement actions include a quarantine ACL being applied to the endpoint.

   For NADs using NAC-L2-802.1x, the enforcement action includes a quarantine VLAN.

   - Enforce Redirection (optional)—In this example, the endpoint device is assigned a URL redirect to the remediation server.
   
   

6. The NAD receives the quarantine policy enforcement from Cisco Secure ACS. It quarantines the endpoint and sends the endpoint a redirect URL to go to the remediation server.
7. The endpoint is now quarantined and redirected to a remediation server. With NAH, the URL redirect is the only way to provide feedback to the user because there is no posture agent present. At this point, the user can elect to do nothing and remain quarantined, or comply and allow their host to remediate by installing Cisco Trust Agent.

From this point, the NAC Framework process is the same as the example in which the endpoint state changed from quarantine to healthy as shown in Figure 6-3.